The invention relates to a method for producing thin coatings and to an apparatus for the practice of the method.
Many different methods are known in the field of physical and chemical depositing technologies for the production of thin coatings. Different methods are used according to the desired properties of the coating to be deposited and on the material systems chosen.
The method of cathode sputtering is advantageous, especially for materials of high melting point, in which a plasma is ignited in a vacuum in a range that is clearly above any typical residual gas pressure for vapor depositing processes, by using an electrical field from which ions are accelerated against a target that is at a high electrical cathode potential, and these ions knock atoms out of the target which then deposit themselves on the walls of the vacuum chamber and on a substrate usually at ground potential or at a low bias voltage, which is situated at a distance from the target. Heating of the material source is not necessary, and instead the target is cooled during the process. The result is a residual gas pressure usually mostly of an inert gas such as, say, argon which has no unwanted influence on the coating that forms on the substrate. For the deposition of compounds such as nitrides, carbides or oxides or the like, appropriate reactive gases can be admixed additionally to the sputtering gas.
The substrate is usually arranged outside of the plasma zone in order to prevent any damage to the freshly growing coating by radiation from the plasma or by residual sputtering effects. The average free length of travel of the ions must be great enough so that they can reach the target with sufficient kinetic energy, i.e., with minimum interference due to further collision processes in the residual gas, which sets a maximum limit on the possible residual gas pressure. On the other hand, the pressure must be high enough to be able to ignite a stable plasma. With magnetic field-supported cathode sputtering it is possible to produce an elevated electron density on the target, resulting in a high plasma density at the target and therefore a greatly elevated sputtering rate.
By the addition of reactive components, especially oxygen, to the inert gas, oxides can also be produced. Such a reactive sputtering process is disclosed, for example, in WO 01/73151 A1, where the oxygen partial pressure during the sputtering of the oxide must be controlled by means of a lambda probe, so that a stoichiometric oxide can form in the growing coating. Of course, the target also reacts with the reactive gas, so that competing processes, namely ablation on the one hand, and the formation of oxide on the target surface to inhibit the ablation. This in turn has repercussions on the electrical potential in the coating chamber, the formation of plasma, and the like. Likewise, the coatings of the sputtered material also form, getter surfaces which bind oxygen, for example, as a reactive component and thus lead to a mutual, hard to predict interdependence of a variety of process parameters. Here too the relationship among the coating parameters is very complex. Often there is then a mutual influence when just one coating parameter is varied. Depending on the coating material to be deposited, it is therefore necessary to attune the coating processes and the coating parameters to one another. This is all the more true the more complex a layer system to be deposited is, say, in the case of the deposition of multiple layers having special functional properties, especially optical function coatings. The problems mentioned are especially pronounced in the so-called reactive DC magnetron sputtering of metallic compounds, in which the requirement of a reacting compound on the substrate surface in the case of a metallic target surface can be achieved only with great expense. For the production of insulating coatings, such as, e.g., SiO2, Al2O3 and the like, methods have therefore already been developed in which, by means of two pairs of magnetron sputtering cathodes supplied by an alternating current source, two targets are used in alternation. The polarities of the target potentials usually vary in the kHz range, i.e., each cathode is alternately cathode and anode. This leads to a definite charge transport between cathode and anode without the hampering effect of an oxide coating on the target surfaces, in contrast to the disturbing effect of the so-called “disappearing anode” in the case of reactive DC magnetron sputtering.
Efficient operation, however, requires operating in the so-called transition area since otherwise the formation of oxide on the target surface is faster than the ablation rate.
EP 0 795 623 A1 discloses an apparatus for the application of thin titanium oxide coatings by reactive cathode sputtering. Accordingly the power supply to the cathode is regulated by the signal from a y probe sensor which compares the content of oxygen in the vacuum chamber with a reference gas. The method is especially suited to the long stable deposition of oxides, which are to be made as uniform as possible, with an unvarying composition.
DE 42 36 264 C1 discloses a plasma-supported electron beam vapor deposition in which an oxide is vaporized at a very high rate by an electron beam vaporizer and deposited on a substrate. During the vaporization, however, the oxide dissociates so that the oxygen is lost and is no longer available for oxidation in the growing coating. Between substrate and vaporization source there is therefore a plasma space containing an oxygen plasma, in which the vapor is excited on the way to the substrate, so that a stoichiometric oxide can deposit itself on the substrate. Depending on the material system, the deposition of a stoichiometric oxide is successful since either the partial pressure of the reactive gas or the plasma parameters are regulated during the coating process.
The relationships are very complex and can hardly be transferred from one material system to another. Variation of individual process parameters produces different results in different material systems. Deposition parameters optimized for aluminum oxide, for example, do not yield optimum results, in the case of silicon oxide, for example. Moreover, different vaporization parameters which can not be ascertained separately appear also within one and the same material system, which lead to undesired alterations of the properties of the deposited coatings and make the repeatability of a started coating process additionally difficult.
In EP 0 1516 436 E1 a magnetron sputtering apparatus is disclosed for the reactive depositing of a material onto a substrate with a magnetron sputtering apparatus and a secondary plasma apparatus. The sputtering system and the secondary plasma apparatus have each sputtering and activation zones which are atmospherically and physically adjacent. By bringing together the sputtering and activation zones, the plasmas of both zones are mixed to form a single, continuous plasma.
In EP 0 716 180 B1 a coating apparatus is disclosed, with a deposition system and an apparatus for producing a plasma. The deposition and plasma apparatus can be operated selectively, so that a composition layer is formed which has at least several layers. The composition of each layer can be chosen from at least one of the following substrates: a first metal, a second metal, an oxide of the first metal, an oxide of the second metal, mixtures of the first and second metal and oxides of mixtures of the first and second metal.